Months before the 2013 terror attack on the Boston Marathon, accused bomber Tamerlan Tsarnaev posed in front of a black flag often associated with jihad with a fellow worshiper at a Boston-area mosque, according to an FBI report obtained by ABC News.

The FBI report, which did not include the photo, describes Tsarnaev and his friend Khairullozhon Matanov as “seated in front of a black flag with a sword and a shadada phrase,” referring to the Muslim statement of faith, and adds that the photo was taken “at the mosque.” Similar flags have become symbols of jihad, used by al Qaeda and al Qaeda-linked extremist groups.

The FBI had been warned by Russian intelligence in 2011 that Tsarnaev may have become radicalized, but the bureau dropped its inquiry months before the photo was taken because it said it “did not find any terrorism activity.”

The black flag picture is part of the evidence prosecutors said Matanov deleted from his computer to obstruct the investigation into the bombings, which in turn led to the arrest of the Kyrgyzstani national last month on federal charges. He has pleaded not guilty and is currently being held without bail.

The FBI report says the alarming photograph was taken on Eid-Al-Adha, also known as the Festival of Sacrifice, the highest of Muslim holidays, but does not say exactly when. A source familiar with the investigation told ABC News it was taken during the August 2012 celebration. The FBI report does not identify the mosque where the photograph was taken.

Tsarnaev and Matanov prayed at a mosque in Cambridge, Mass., the FBI report says, but Yusufi Vali, the Executive Director of the Islamic Society of Boston that opened the mosque told ABC News there were no reports of the black flag of jihad being at the 2012 Eid holiday, or at any other time. Tsarnaev had been asked to leave the Cambridge mosque after he disrupted services later that same year.

“Without doubt there is no symbol of violence or terrorism at the [Cambridge] center," said Vali. “I can confidently say that we preach moderation in line with more American values."

The Islamic Society of Boston opened its first mosque in 1994 on Prospect Street in Cambridge in a former Knights of Columbus building. A larger $15.6 million Roxbury mosque opened in 2009. Because of its size, high holidays, like Eid, are celebrated there, Vali said.

Before Tamerlan Tsarnaev began to pray at the Cambridge mosque, which he did more frequently than his younger brother and alleged Boston bombing co-conspirator Dzhokhar, the Islamic Society of Boston over the years had had a small number of run-ins with alleged would-be violent extremists.

The Islamic Society of Boston was founded by Abdulrahman Alamoudi, who pleaded guilty in 2004 to charges related to his “activities... with nations and organizations that have ties to terrorism” -- including a link to an assassination plot targeting Saudi Arabian Crown Prince Abdullah ibn Abdilaziz, according to the Department of Justice. He was sentenced to 23 years in prison. To this day the FBI lists Alamoudi’s prosecution as a “Major Terrorism Case” on its website.